1. Field of the Disclosure
The present disclosure relates generally to an optical scanning system in an imaging apparatus, and more particularly to a compact collimation assembly incorporating multiple light sources and collimation lenses for use in over-filled scanner (OFS) scanning system.
2. Description of the Related Art
In various imaging devices which utilize light to form images, optical scanning systems are typically incorporated to scan light beams from one or more light sources onto a target image plane surface. In an electrophotographic imaging device, for example, the image plane surface is typically a photosensitive member. Generally, light beams are swept across the image plane surface by a scanning mirror to form light spots upon the image plane surface along a scan line direction. Commonly used scanning mirrors include rotating polygon mirrors which scan light beams in one direction.
A polygon mirror can have either an under-filled or over-filled facet design. In an under-filled design, the facet length is significantly wider than the incident light beam width such that the beam footprint on a facet never crosses over the edges of the facet from start to end of a scan line operation. On the other hand, an over-filled design has a facet length that is narrower than the incident light beam such that the beam footprint on a facet completely fills the facet and extends beyond its edges over the duration of a scan line operation. In this case, the width of the laser beam after it is reflected by the polygon mirror is determined by the size of the polygon facet.
Generally, in order to have a decent optical performance particularly on laser spot size, the width of a light beam striking a polygon facet must be at least some requisite value, such as 4 mm. By comparison, for a given number of polygon facets, the under-filled design would require a larger polygon diameter since size of a facet would have to be wider than the requisite beam width, while the over-filled design would require a smaller polygon diameter since length of a facet only needs to be at least the same as the requisite beam width. Thus, scanning systems that employ polygon mirrors with larger number of facets can be implemented at lower costs using the overfilled design. In addition, polygon mirrors having smaller diameters are not only significantly less expensive, but also run faster, have less acoustic noise and contamination on the polygon facets, and allows faster time to first print.
In color imaging systems, one of the challenges of having an over-filled facet design is to achieve a sufficiently wide incoming beam with relatively small wavefront error for good beam quality for all four color channels. In some existing approaches, beam expanding optic sets have been used to expand laser beams along a scan direction. As an example, FIGS. 1A-1B illustrate an optical layout of a known scanning system 10 employing an over-filled polygon facet design. FIG. 1A is top view of the optical layout of scanning system 10 and FIG. 1B is a side view thereof. It is noted that prescan mirrors have been removed in FIG. 1B to more clearly illustrate the tracing of beams. Four light beams 15 from four light sources 20, each for a different color channel, are collimated through four collimation lenses 25 so that each light beam 15 propagates with a constant beam shape and size. After passing through collimation lenses 25, the four light beams 15 are received by respective prescan mirrors 30 and combined to share the same downstream prescan optics before reaching a polygon mirror 35, as shown in FIG. 1A, while remaining separated along the cross-scan direction 40 as shown in FIG. 1B. The downstream prescan optics include four prescan lenses 45A-45D, and two additional prescan mirrors 50A, 50B. First prescan lens 45A and third prescan lens 45C are cylindrical lenses with optical power along the cross-scan axis to converge the four light beams 15 along the cross scan direction 40. In order to expand each light beam 15 after collimation lenses 25, second prescan lens 45B typically has a spherical concave surface so as to diverge each light beam 15 along a scan direction perpendicular to the cross-scan direction 40. Meanwhile, fourth prescan lens 45D is a cylindrical lens with optical power along the scan axis so as to collimate and slightly converge each light beam 15 along the scan direction. Accordingly, each light beam 15 arrives at polygon mirror 35 with a sufficient beam width that overfills a facet of polygon mirror 35 as shown in FIG. 1A.
However, in the example design illustrated in FIGS. 1A-1B, the optical layout includes a relatively large number of optical components which presents added complexity and cost to the scanning system 10. Moreover, the design requiring six prescan mirrors 30, 50 and four prescan lenses 45A-45D before the light beam reaches the polygon mirror 35 reduces robustness of the scanning system. This is because optical performance of a scanning system is generally very sensitive to alignment of the optics upstream of the scanning mirror. By having a larger number of optical components before the scanning mirror, additional accumulated tolerances are introduced on the optical path which makes it difficult to have precise optical alignment. Additionally, in order to maintain alignment accuracy, most of the prescan mirrors 30, 50 have mechanical features to allow for tilt angle adjustments along both scan and cross-scan directions which may not only add more cost but also reduce the overall system robustness.